Normally, you would attatch minimum of 1" grey PVC electrical conduit to the bottom of a knockout on your main, bury the pipe using sweeps or a heat gun, and run it to your subpanel.
BUT, you can't add another 100 amps service onto your main if it's only 100 to begin with. You better check that first. If your wire size coming in to your main panel, and the panel itself is set up for it, sometimes you can just change the main breaker on the panel to a larger amperage.
If you're not sure about this, cough up the dinero and pay an electrician, better to be out a little bit of money than have the house burn down or electrocute yourself!
Good luck - Karl.

There is a lot to consider when installing a subpanel. First, off you need to do what is called a demand load calculation to find out what you really need in the garage. I have a 100 amp panel in the garage as subpanel fed by a 60 amp breaker in the main box. 60 amps is all that I needed and I have a two stage compressor, mig welder, several lighting circuits, receptacles around the walls etc. A one man shop can only use one tool at a time. Code requires a detached garage to be fed by a 4 wire feeder for 230 volts - 2 hots, one nuetral, and a ground.

There is a lot to consider when installing a subpanel. First, off you need to do what is called a demand load calculation to find out what you really need in the garage. I have a 100 amp panel in the garage as subpanel fed by a 60 amp breaker in the main box. 60 amps is all that I needed and I have a two stage compressor, mig welder, several lighting circuits, receptacles around the walls etc. A one man shop can only use one tool at a time. Code requires a detached garage to be fed by a 4 wire feeder for 230 volts - 2 hots, one nuetral, and a ground.

Yep, load calculations are very important. My Esab plasma cutter alone could draw close to 70 amps if cutting heavy metal. Generally calculating the amperage draw of all the equipment you would most likely ever have on at one time will give you a good idea of how big a subpanel you really need.
Later - Karl.

ok, i looked at the panel when i got home, it has two 125 amp breakers for the main breaker, and there are a couple slots left. so i should be able to tie it in, i think. i want to build the system to allow for the largest load i could possibly put on it. a bunch of lights, an air conditioner, and a plasma cutter could put me close to 70 or 80 amps.

i know about the load function, i was more concerned with how to actually tie in the wires to the box and hook up the subpanel, than what size breakers i need.

Mine panel was bought at Lowes and is a GE "garage pak". It comes with the green bonding screw not installed and was not used since I have a four wire feeder. The panel came with a main breaker and also 5 20 amp breakers. I purchased the other breakers for compressor etc.

know about the load function, i was more concerned with how to actually tie in the wires to the box and hook up the subpanel, than what size breakers i need.

Before you start adding panels, I'd check into local codes and your insurance policies. Many companies have policies that say you need an electrician to install such items, or you may void your insurance.

I upgraded my old house (not the one I am in now) from 100 to 200 amps so I could run an air compressor, fridge and tv in the garage at the same time -- plus the big window air conditioner kept kicking a breaker because we didn;t have enough circuits upstairs. It only cost a few hundred bucks.

sounds like you are set... but for anyone else, figure out how much juice you think you need and multiply by two, there will always be more to add and you never take anything out....

Estimates I received were way more than a few hundred bucks. I don't know what your codes are to your specific area, but for me it ment moving the meter out of the basement to an outside wall (new service to the house, weatherhead, conduit to get power from meter to the new panel) all circuits have to be grounded, separate circuits for dishwasher, micro wave, any receptacle within 4' of sink has to be GFCI. You can not buy the materials for a few hundred, then the labor- if your house is anything like mine most circuits would be impossible to access without cutting up some drywall.

my panel is on an exterior wall, right in front of the garage, so that is not an issue, my next problem is getting across the driveway.

To update a panel from 100 amp to 200 amp requires a new feeder from the transformer to the panel, it is very unlikely that the feeders you have now will support 200 amp. At least where I am, to get the feeders updated you have to pull a permit, that means an electrician, and when that is issued it will be required that you update the house to current code. If your house is new enough the update should be minimal.

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.

User Name:

Password

Please enter a password for your user account. Note that passwords are case-sensitive.

Password:

Confirm Password:

Email Address

Please enter a valid email address for yourself.

Email Address:

Log-in

User Name

Remember Me?

Password

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.